Rail fittings have been employed in the marine industry, particularly as associated with pleasure boating, to mount various equipment and structures to rails. Since typical rails are more or less continuous lengths of tubing, suitable rail fittings must be constructed so as to be attachable to and detachable from a rail without having to dismantle the rail. Moreover, suitable rail fittings should be constructed so that their position along a rail may be adjusted without having to dismantle the fitting and without having to remove whatever is attached to the fitting. Rail fittings of the category contemplated by the present invention are useful in installing so-called "bimini" tops, stringers for fender racks, and other similar uses where a "tube slide" or "rail mount hinge" would be employable, and are useful in installing CB, radar, fishing and other apparatus to rails.